1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on a MR (magnetic resonance) signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which image substances showing mutually different chemical shifts selectively by adjusting a center frequency of an excitation pulse
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) is an imaging method which excite nuclear spin of an object set in a static magnetic field with a RF signal having the Larmor frequency magnetically and reconstruct an image based on a MR signal generated due to the excitation.
In the field of magnetic resonance imaging, chemical shift imaging for selectively imaging a specific substance is performed taking advantage of the variation in chemical shift from one substance constituting an object to another. The chemical shift is a variation in magnetic resonant frequency occurring due to the screening effect of orbital electrons. In view of examples of water and fat representative of the tissue of a living body, it is known that the resonant frequency of fat is shifted by about 3.5 ppm (parts per million) with respect to the resonant frequency of water (0 ppm in chemical shift).
This makes it possible to perform imaging by the fat suppression method for frequency-selectively suppressing signals from fat, or the water suppression method for frequency-selectively suppressing signals from water, by taking advantage of the difference in the center frequency for magnetic resonance. However, the peak of resonant frequency based on a chemical shift is influenced by the uniformity of a static magnetic field intensity, and if the uniformity of the static magnetic field intensity is imperfect, the peak of resonant frequency is also imperfect, thereby making it difficult to identify a substance. Hence, in the imaging by the fat suppression method or water suppression method taking advantage of chemical shift, shimming for adjusting the uniformity of the static magnetic field intensity is performed prior to the imaging in order to make clear the peak of resonant frequency in response to a substance.
Now, an attempt to selectively image not only fat but also silicone used as a material of a breast implant, by the chemical shift imaging, is under way. The materials used as a breast implant include saline in addition to silicone. In recent years, an influence of the leakage of silicone upon a normal mammary tissue is being turned into problem, and a saline implant having higher safety has been used more frequently. The interest of patients is, therefore, being focused on whether the material for an implant is silicone or saline.
However, an appropriate examination method for determining whether the implant is constituted of silicone or saline has not yet been established. As a result, a complicated examination method is now used wherein imaging such that the center frequency of excitation pulse is set to the resonant frequency of saline or water is performed to selectively image saline or water, and thereafter, imaging such that the center frequency of excitation pulse is set to the resonant frequency of silicone to selectively image silicone. In other words, the breast implant imaging requires a water image obtained by frequency-selectively enhancing signals from water, and a silicone image obtained by frequency-selectively enhancing signals from silicon.
FIG. 23 is a flowchart showing a conventional procedure of a chemical shift imaging of water and silicone, wherein symbols S each denote a step in the flowchart.
First, in step S1, first-time shimming is performed by an operation by an operator, and the uniformity of the static magnetic intensity is adjusted. Specifically, a current to be supplied to a shim coil provided for adjusting the static magnetic intensity is controlled.
Next, in step S2, a scan is performed for acquiring a frequency spectrum representing the resonant frequency of each substance based on a chemical shift. The frequency spectrum acquired by this scan allows the peak of the resonant frequency of water or saline to be detected.
Then, in step S3, the center frequency of excitation pulse for imaging is set to the resonant frequency of saline or water. The chemical shift of the resonant frequency of saline is 0.2 ppm, and hence, even if signals from water, of which the chemical shift in resonant frequency is 0 ppm, are selectively imaged, whether the implant is silicone or saline can be determined. Therefore, for example, the center frequency of excitation pulse is set to the resonant frequency of water.
Sometimes a series of work from the adjustment of the uniformity of static magnetic intensity up to the setting of the center frequency of excitation pulse is referred to as “shimming”, but here, adjustment work of the uniformity of static magnetic intensity is designated as “shimming”.
Next, in step S4, imaging is performed with the center frequency of excitation pulse set to the resonant frequency of water. This allows a water image obtained by frequency-selectively enhancing signals from water to be acquired.
Next, it is necessary to perform imaging in which the center frequency of excitation pulse is set to the resonant frequency of silicone. Here, the chemical shift of the resonant frequency of silicone is about −5 ppm with respect the resonant frequency of water (0 ppm in chemical shift). Accordingly, based on the frequency spectrum already acquired, the center frequency of excitation pulse can be set to the peak of the resonant frequency of silicone. However, some apparatus cannot change the center frequency of excitation pulse unless shimming and a scan for frequency spectrum acquisition are performed.
For such an apparatus, a second-time shimming is performed by an operation by the operator in step S5.
Then, in step S6, a scan for acquiring a frequency spectrum is performed. From the frequency spectrum obtained by this scan, a peak of the resonant frequency of silicone can be detected.
Next, in step S7, the center frequency of excitation pulse is set to the resonant frequency of silicone.
Then, in step S8, imaging is performed with the center frequency of excitation pulse set to the resonant frequency of silicone. This allows silicone image obtained by frequency-selectively enhancing signals from silicone to be acquired.
On the other hand, in order to improve the fat suppression effect utilizing chemical shift, a PASTA (polarity altered spectral-spatial selective acquisition) sequence has been devised for performing imaging by using in combination 90° RF pulse for proton excitation and 180° RF pulse for refocus, and making mutually opposite the polarities of first and second gradient magnetic fields for slice selection, the first and second gradient magnetic fields being applied along with the 90° RF pulse and 180° RF pulse, respectively (for example, refer to Japanese Patent Application (Laid-Open) No. 9-122101; or Miyazaki M, Takai H, Tokunaga Y, Hoshino T, and Hanawa M: A polarity altered spectral and spatial acquisition technique in “Proceedings, ISMRM, 3rd Annual Meeting” Nice, p. 657, 1995).
That is, the imaging by the PASTA sequence employs the first gradient magnetic field for slice selection, applied along with 90° RF pulse, and the second gradient magnetic field for slice selection, applied along with the 180° RF pulse. The second gradient magnetic field is made opposite in polarity to the first gradient magnetic field. Here, the frequency bandwidth of 90° RF pulse is set to a narrow width such as to prevent the resonant frequency band of water and that of fat from overlap each other there, that is, such as to be able to select water and fat utilizing chemical shifts. Conversely, the frequency bandwidth of the 180° RF pulse is set to a width such as to be able to refocus both of water and fat.
When, for example, fat suppression is performed using such a PASTA sequence, the protons in a water portion, excited by the 90° RF pulse are refocused by the 180° RF pulse under the gradient magnetic field having a polarity opposite to that in the excitation mode. As a result, echo signals occur from the protons in the water portion. On the other hand, the protons in a fat portion are refocused by the 180° RF pulse without being affected by the 90° RF pulse. Consequently, no echo signal occurs from the protons in the fat portion. This allows an achievement of a water image by fat suppression.
However, in the conventional chemical shift imaging, when attempting to selectively image each one of a plurality of different substances by using advantage of differences in chemical shift, it is necessary to preliminarily perform complicated work such as shimming, a scan for a frequency spectrum, an adjustment of the center frequency of excitation pulse by the operator, for each of the substances every imaging. For example, as described above, the operator must perform each of the shimming for selective acquisition and a scan for frequency spectrum acquisition regarding water signals, and shimming for selective acquisition and a scan for frequency spectrum acquisition regarding fat signals or silicone signals. In addition to the shimming, the operator must change the center frequency of excitation pulse each time the operator performs imaging about substances mutually different in chemical shift. For example, in the imaging of a breast implant, the operator must change the center frequency of excitation pulse from the resonant frequency of water into that of silicone.
Particularly when the operator without knowledge of chemical shift operates the apparatus, there is a possibility that proper shimming and/or the setting of the center frequency of excitation pulse may be difficult, with the result that an intended image may be unable to be obtained. Conversely, when the operator performs imaging utilizing chemical shift, there occurs a need for the operator to acquire knowledge thereof, which becomes a factor responsible for reduction in convenience of the magnetic resonance imaging apparatus. Furthermore, the imaging of molecules of drug or the like requires very detailed knowledge of chemical shift, and hence, under the current circumstances, substances that can be treated as targets of chemical shift imaging are limited to specific substances such as water, fat and the like.
Also, when shimming is not properly performed, or when a peak of frequency spectrum is detected by the operator without sufficient knowledge of chemical shift, an erroneous frequency is recognized as a resonant frequency, thereby causing a problem in that an improper frequency may be erroneously set as the center frequency of excitation pulse.